So where is the ground in this circuit?

Thread Starter

Denesius

Joined Feb 5, 2014
119
This is a circuit for a military gyro* that I'm trying to understand. Power input is 28vDC at A & B on the upper left. Output is 3-phase 400Hz AC (28v, I believe) on G, H & K at the bottom. VR1 is a 12v Zener. The emitters of Q6, 7 & 8 are tied to the collector of Q2, which thru a fast diode goes to input ground. Chassis ground is separate from everything, and the circuit ground has no direct connection to anything tied to input ground. Connection N on the bottom, tied to the collector of Q2, is open.
I've figured out that L1 & L2 and associated capacitors are for noise suppression. What is the source of the ground in the main circuit and the power supply (lead 3) of the opamps? What is the purpose of C1 capacitor on the left off of the main circuit board, and does it have any role in establishing the power rails for the main circuit?
(B1 is a warning flag motor & can be ignored; VR2-5 are protection diodes for the lamp circuit, and again irrelevant to the question)
Any thoughts on what's going on here would be greatly appreciated.
Thank you all.
Schematic B-submit.jpg
*-BTW I'm trying to fix this thing, and if anyone knows of a source for the U2T101 and U2T151 darlingtons, well that would get you even more positive mojos!
 

MrChips

Joined Oct 2, 2009
24,626
Here are accepted ground symbols:

1633443047395.png

In your circuit, DC power comes in at (A) 28VDC and (B) PWR GND.

Opamps U1, U2, U3 pin-3 are connected to Signal GND.
The return path to PWR GND is via R7, Q2 and CR1.

L1, L2, C1, C2 are noise suppression filters that returns EMI to chassis ground.
 

iggnator

Joined Jan 30, 2019
9
This is a circuit for a military gyro* that I'm trying to understand. Power input is 28vDC at A & B on the upper left. Output is 3-phase 400Hz AC (28v, I believe) on G, H & K at the bottom. VR1 is a 12v Zener. The emitters of Q6, 7 & 8 are tied to the collector of Q2, which thru a fast diode goes to input ground. Chassis ground is separate from everything, and the circuit ground has no direct connection to anything tied to input ground. Connection N on the bottom, tied to the collector of Q2, is open.
I've figured out that L1 & L2 and associated capacitors are for noise suppression. What is the source of the ground in the main circuit and the power supply (lead 3) of the opamps? What is the purpose of C1 capacitor on the left off of the main circuit board, and does it have any role in establishing the power rails for the main circuit?
(B1 is a warning flag motor & can be ignored; VR2-5 are protection diodes for the lamp circuit, and again irrelevant to the question)
Any thoughts on what's going on here would be greatly appreciated.
Thank you all.
View attachment 249595
*-BTW I'm trying to fix this thing, and if anyone knows of a source for the U2T101 and U2T151 darlingtons, well that would get you even more positive mojos!
This looks like a partial schematic. Note the dashed line around the main circuit, with labeled connections leaving it. The signal ground has to be on another page.
 

Thread Starter

Denesius

Joined Feb 5, 2014
119
Opamps U1, U2, U3 pin-3 are connected to Signal GND.
The return path to PWR GND is via R7, Q2 and CR1.
I wondered about that, but R7 is 2.87K. Isn't that a rather high resistance ground for the opamps?

This looks like a partial schematic. Note the dashed line around the main circuit, with labeled connections leaving it. The signal ground has to be on another page.
Nope, this is the whole thing. The dashed lines represent the circuit board. Everything else is mounted off. There are pinouts on the circuit board, some are connected as noted, others are open.

Do you see all of the little triangles pointing toward the bottom of the page? Those are the ground connections, that is how you do it in a schematic.

http://lednique.com/ground-earth-chassis/
Thank you, but I didn't make it clear. The question was: how do the signal grounds (the downward triangles) interact with power ground (the input 28VDC). Right now I don't see a return path.
 
Last edited:

iggnator

Joined Jan 30, 2019
9
Nope, this is the whole thing. The dashed lines represent the circuit board. Everything else is mounted off. There are pinouts on the circuit board, some are connected as noted, others are open.
Does an ohm check show signal ground and PWR ground the same? This is pretty old, how many layers are the PC board? Gyro's are pretty much dead technology these days. (I worked for Collins Radio, and they were dead when I started in 1984, we still made them but for old aircraft installs, and upgraded to new technology was ongoing. I know there were other vendors that made panel mount backup attitude systems using gyros, but we were not in that market).
 

Thread Starter

Denesius

Joined Feb 5, 2014
119
Does an ohm check show signal ground and PWR ground the same? This is pretty old, how many layers are the PC board? Gyro's are pretty much dead technology these days. (I worked for Collins Radio, and they were dead when I started in 1984, we still made them but for old aircraft installs, and upgraded to new technology was ongoing. I know there were other vendors that made panel mount backup attitude systems using gyros, but we were not in that market).
It's 70's technology. The gyro is brand new, but the inverter board is bad. I don't plan on aircraft installation... the thing weighs about 5 pounds!
If you want to call it a "PC board", it's two layers. The reason for the quotes: there are a few traces and many connections using extended component leads!
There is no connection between signal and power ground on the schematic. I just checked with ohmmeter and it shows high resistance (1K+) with an obvious capacitance somewhere in the circuit.
I'm basically trying to understand how the thing works: is the circuit around Q1 & Q2 some kind of oscillator, or does it split the 28v input to provide the +/- rails for the opamps?
 

Thread Starter

Denesius

Joined Feb 5, 2014
119
Sorry, I misread the component values.

R7 provides the path from Signal GND to PWR GND.
Op-amp -ve supply voltage is via Q6, Q7, and Q8.
I would say the opamp power rails are via Q6, 7, and 8 on the negative rail, and Q 3, 4 and 5 on the positive rail. And I think the circuit around Q1 and Q2 split the input power for the rails. In my limited knowledge, the only connection I see between the signal ground and power is via R6 (2.87K) to the positive, and via R7 (2.87K) and Q2 and CR1 to the negative.
Voltage splitter?
Oscillator?
 

MrChips

Joined Oct 2, 2009
24,626
I would say the opamp power rails are via Q6, 7, and 8 on the negative rail, and Q 3, 4 and 5 on the positive rail. And I think the circuit around Q1 and Q2 split the input power for the rails. In my limited knowledge, the only connection I see between the signal ground and power is via R6 (2.87K) to the positive, and via R7 (2.87K) and Q2 and CR1 to the negative.
Voltage splitter?
Oscillator?
Yes, R6 and R7 constitute a voltage splitter to create a pseudo-ground, i.e. a reference voltage that is halfway between the power supply rails. It does not need to be of low impedance because the inputs of the op-amps are high impedance and do not take a lot of current.
 

iggnator

Joined Jan 30, 2019
9
I would say the opamp power rails are via Q6, 7, and 8 on the negative rail, and Q 3, 4 and 5 on the positive rail. And I think the circuit around Q1 and Q2 split the input power for the rails. In my limited knowledge, the only connection I see between the signal ground and power is via R6 (2.87K) to the positive, and via R7 (2.87K) and Q2 and CR1 to the negative.
Voltage splitter?
Oscillator?
Yes, VR1 with Q1, Q2 are there to prevent the power across Nodes N and M (these must be test points on the board) from exceeding 36 volts per note 3. This is a standard qualification test requirement on 28VDC avionic generator systems. So it looks like they put in a circuit that raises what would be the 'negative' rail as the input 28VDC exceeds 36VDC.
This is a very odd circuit. I don't see the feed back resistors for the 3 opamps, just the input resistors R5, 14, 21. Odd to use the opamp power pins to drive the push pull transistors. I think they get the 120degree phase shift for the 3phase power to the gyro motor from C5, 6, 7 along with R5, 14, 21.
I was always amazed at the clever circuit design when I first started work, they always gave the new kids collateral work on the previous generation. In my case it was the autopilot computer, an all analog design using attitude inputs from 3phase 440Hz synchro outputs from the gyro platform.
 

MrChips

Joined Oct 2, 2009
24,626
The three op-amps are configured as analog comparators.
The three comparators are three inverters in a cascaded loop to form a 3-phase oscillator, each 120° apart.
 

Thread Starter

Denesius

Joined Feb 5, 2014
119
I was always amazed at the clever circuit design when I first started work, they always gave the new kids collateral work on the previous generation. In my case it was the autopilot computer, an all analog design using attitude inputs from 3phase 440Hz synchro outputs from the gyro platform.
On an aside note: surreptitiously I met an older guy online that was involved with the development of the Bonzer radar altimeter. They were working on an unrelated line of sight GHZ transceiver when they noticed passage of cars in line with the antenna was causing a phase shift. Put a couple of young guys on the project idea: Low & behold a cheap certified radar altimeter! We're talking early 70's point to point wiring circuits in a gigahertz transceiver. Amazing! I have their schematic and a full working unit, if anyone's interested.
 

Thread Starter

Denesius

Joined Feb 5, 2014
119
BTW: Thank you one & all. Whether solutions are discovered or not, it's always an educational experience on this site.
 
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